Long-Term Effects Of Fertilization On Chemical Properties Of Soil Organic Carbon In Two Typical Agricultural Soils

Soil organic carbon(SOC)is an important part of the organic carbon in terrestrial ecosystems,which the chemical properties are mainly determined by its chemical structure and composition.Soil organic carbon has a high degree of heterogeneity among different soil components as well as the chemical properties also have certain differences.In this paper,two typical farmland soils from the long-term localized fertilization experiment in China were used as the research object.The soil was divided into wide particle(>0.25mm),small particle(0.053-0.25mm)and mineral-bound fractions(<0.053mm)by using the particle-size group method.The carbon and nitrogen contents and organic carbon chemical composition of different components under different fertilization treatments were measured to comprehensively analyze the dynamic mechanism of the internal chemical properties of soil organic carbon.The main results obtained are as follows:1.In different treatments,both fluvo-aquic soil and red soil showed that the content of mineral-bound organic carbon was higher than that of small particle and wide particle organic carbon.The proportion of mineral-bound organic carbon in fluvo-aquic soils was the highest,reaching over 70%,followed by small particle components,and the wide particle components were the lowest.The ratio of mineral-bound components in red soils is more than 80%.The content of wide particle organic carbon components is the second,and the content of small particle organic carbon components is the lowest.2.The fertilization treatment increased the total nitrogen content in fluvo-aquic soil and red soil.There was no significant effect that the total carbon content in fluvo-aquic soil and red soil treated with nitrogen fertilizer compared with non-fertilizer treatment(P>0.05).C/N of different treatments in fluvo-aquic soil showed that non-fertilizer(CK)> single application of nitrogen fertilizer(N)> combined application of NPK(NPK)> organic and inorganic fertilizer application(NPKM).However,the ratios of C/N in red soil was just the opposite.The result showed that organic and inorganic fertilizers combined(NPKM)> NPK combined application(NPK)> non-fertilizer(CK)> single application of N fertilizer(N).It is due to the single application of nitrogen fertilizer resulted in serious soil acidification and decreased in the total carbon content,while the total nitrogen content was less affected,so as to lower C/N ratio.In addition,the total carbon content of the soil after fertilization applied was increased,while the serious nitrogen leaching in the red soil region resulted in the total C/N ratio lower than CK.All the different fertilization treatments in fluvo-aquic soils showed that the carbon and nitrogen contents of the small particle components were lower than those of the other two components,indicating that the small particle components were difficult to bind with carbon and nitrogen.C/N also showed the lowest content of small particle components and the highest content of wide particle components,indicating that the wide particle components had a low degree of soil decomposition.Total carbon content of the red soil in the NPKM treatment was affected by the application of organic fertilizer and showed that the total carbon content of the small particle components was higher than that of the wide particle components,while the rest decreased with the decrease of the soil particle size.In addition,the soil C/N ratio also decreased with the decrease of the soil particle size,indicating that the wide particle components had the lowest degree of soil decomposition,and the mineral-bound components had a high degree of soil decomposition.3.The Py-GC/MS results of the two kinds of soil organic carbon indicated that fertilization can increase the content of lignin-derived compounds in soil organic carbon,and the application of organic fertilizers has the most obvious effect.The analysis of the chemical composition of different components of organic carbon showed that lignin compounds,polysaccharide compounds,and aliphatic compounds derived from microorganisms in fluvo-aquic soils were accumulate in large amounts in wide particle organic carbon components,while plant-derived alkanes and olefins were accumulated in the mineral-bound organic carbon components.The content of lignin compounds is substantially zero or very low in the mineral-bound organic carbon component.The lignin compounds also showed accumulation in wide particle organic carbon in the red soil of Qiyang.In addition to NPKM,mineral-bound organic carbon still contained some lignin compounds,while the other mineral-bound organic carbons in other treatments were not contained lignin compounds.The phenomenon of the distribution of polysaccharides and aliphatic compounds is not obvious due to the properties of red soil in Qiyang.And the content of nitrogen-containing compounds less,and no obvious regularity was found.In addition,we can find from the characteristics of the maps in the same type of soil that the profiles of several different treatments in the various components are similar,and there are many kinds of substances in the wide particle organic carbon components and some repeat chemical substances among different treatments.And the similarity of the kinds of each treatment substance in the mineral-bound organic carbon component is high,and the peak characteristics are also relatively consistent,indicating that the organic chemical composition of soil organic carbon is relatively similar under the same degree of decomposition,the degree of decomposition of the mineral-bound organic carbon component is the highest,and the similarity of the chemical composition among the treatments is the strongest.